Measurement of the CKM Matrix Element $\mathrm{\mid }{V}_{cb}\mathrm{\mid }$ from $B^0\to D^{\ast -}{\mathrm{\ell }}^{+}{\nu }_{\mathrm{\ell }}$ at Belle

We present a new measurement of the CKM matrix element $\mathrm{\mid }{V}_{cb}\mathrm{\mid }$ from $B^{0} \to D^{-} \ell^+ \nu_\ell$ decays, reconstructed with the full Belle data set of $711\mathrm{f}{\mathrm{b}}^{-1}$ integrated luminosity. Two form factor parameterizations, originally conceived by the Caprini-Lellouch-Neubert (CLN) and the Boyd, Grinstein and Lebed (BGL) groups, are used to extract the product $\mathcal{F}(1){\eta }_{\mathrm{E}\mathrm{W}}\mathrm{\mid }{V}_{cb}\mathrm{\mid }$ and the decay form factors, where $\mathcal{F}(1)$ is the normalization factor and ${\eta }_{\mathrm{E}\mathrm{W}}$ is a small electroweak correction. In the CLN parameterization we find $$\mathcal{F}$(1)\eta_{\rm EW}|V_{cb}| = (35.06 \pm 0.15 \pm 0.56) \times 10^{-3}$,$\rho^{2}=1.106 \pm 0.031 \pm 0.007$,$R_{1}(1)=1.229 \pm 0.028 \pm 0.009$,$R_{2}(1)=0.852 \pm 0.021 \pm 0.006$. For the BGL parameterization we obtain $$\mathcal{F}$(1)\eta_{\rm EW}|V_{cb}|= (34.93 \pm 0.23 \pm 0.59)\times 10^{-3}$,whichisconsistentwiththeWorldAveragewhencorrectingfor$$\mathcal{F}$(1)\eta_{\rm EW}$\mathrm{.}Thebranchingfractionof$B^{0} \to D^{-} \ell^+ \nu_\ell$ismeasuredtobe$$\mathcal{B}$(B^{0}\rightarrow D^{-}\ell^{+}\nu_{\ell}) = (4.90 \pm 0.02 \pm 0.16)%$. We also present a new test of lepton flavor universality violation in semileptonic $B$ decays, $\frac{{\cal B }(B^0 \to D^{-} e^+ \nu)}{{\cal B }(B^0 \to D^{*-} \mu^+ \nu)} = 1.01 \pm 0.01 \pm 0.03~$. The errors correspond to the statistical and systematic uncertainties respectively. This is the most precise measurement of $\mathcal{F}(1){\eta }_{\mathrm{E}\mathrm{W}}\mathrm{\mid }{V}_{cb}\mathrm{\mid }$ and form factors to date and the first experimental study of the BGL form factor parameterization in an experimental measurement.